In known manner, such a blade is generally designed to be placed together with a plurality of substantially identical blades so as to form a ring around an axis of the ring, which ring has the airfoils disposed substantially radially therein, with the platform surface portions of two adjacent blades situated between their respective airfoils defining an inter-airfoil surface. The inter-airfoil surface connects the pressure side of one airfoil to the suction side of the adjacent airfoil in the inter-airfoil channel.
The set of blades around the ring axis serves to constitute a bladed wheel. The bladed wheel may be a moving wheel and thus receive energy coming from the stream or communicates energy to the stream flowing through the bladed wheel; the wheel may alternatively be stationary, in which case it performs a nozzle function of channeling the stream.
The present invention relates to blades that are used in streams at high temperature, in particular at very high temperature, such as higher than 1000 kelvins (K), for example. This applies for example to the blades located in the high-pressure or low-pressure turbine stages of turbojets that are disposed downstream from the combustion chambers thereof.
These high temperatures (and the associated temperature gradients) can sometimes be much greater than the melting temperature of the blade, and they give rise to various problems for blades: a risk of melting; expansion; deformation; the appearance of mechanical stresses; . . . .
In known manner, the cooling of the airfoils and the platforms of blades that are stressed in this manner is implemented by means of air passages formed within the volume of the blades themselves. These passages convey cooling streams towards those portions of the blade that are subjected to the highest levels of thermal or thermomechanical stress.
The trailing edge of the airfoil, and more particularly the portion where the trailing edge of the airfoil joins the surface of the blade platform, where mechanical stresses are very high, forms a portion of the blade that is most particularly exposed and that is referred to as the critical portion of the blade.
The shape and the location of this portion of the blade make it particularly difficult to cool by means of an air stream. That is why, in practice, the cooling of this critical portion is performed poorly, and it reaches high temperatures simultaneously with high levels of stress. That leads to deformation, and in the long run to cracking, and hence to a reduction in the lifetime of the blade.
The above-mentioned difficulty in cooling the trailing edge of the blade by injecting a stream of air is illustrated in particular in FIG. 3.
FIG. 3 is a section through a blade of the type mentioned in the introduction and it shows the behavior of the cooling streams injected by air injection passages situated in the vicinity of the airfoil of the blade, on its pressure side.
The blade shown in FIG. 3 is a blade 10 having an airfoil 50, a platform 60, and a root 66. The blade is presented in section perpendicular to the longitudinal axis of the airfoil 50.
Air passages 16 are formed in the platform 60. They serve to convey cooling air, which air is injected in particular for the purpose of cooling the critical portion of the airfoil. These passages 16 open out through the blade platform into the platform surface 62, along the pressure side 56 of the blade 10.
It should be observed that in this document, the term “air” is used as a generic term, covering a stream of air or any other essentially gaseous stream, such as for example exhaust gas.
FIG. 3 shows the path followed by air streams injected via the air injection passages 16. These streams do not run along the pressure side wall 56, but instead they rapidly become separated therefrom to follow an oblique path downstream and in part towards the suction side 58 of the adjacent airfoil 50.
Because of this oblique path, these streams contribute little to cooling the portion 12 where the trailing edge joins the platform. Only those cooling streams that are conveyed by the passages 16 located furthest downstream along the airfoil 50 contribute to some extent to cooling this critical portion 12 of the blade 10, albeit imperfectly. The cooling streams conveyed by the other passages 16 move away from the pressure side 56 of the airfoil and do not contribute significantly to cooling the critical portion 12 of the blade.